1. Field of the Invention
The invention relates to a speed reduction mechanism that is suitably used in, for example, an electric vehicle that has an electric motor serving as a driving source, and a motor torque transmission device that includes the speed reduction mechanism.
2. Description of Related Art
There is a conventional motor torque transmission device that is mounted in an automobile, and that includes an electric motor that generates motor torque and a reduction-transmission mechanism that reduces the speed of rotation output from the electric motor and transmits driving force to a differential mechanism (for example, see Japanese Patent Application Publication No. 2007-218407 (JP 2007-218407 A).
The electric motor has a motor shaft that is rotated by electric power from an in-vehicle battery. The motor shaft is arranged along the axis of the reduction-transmission mechanism. Eccentric portions are integrally formed on the outer periphery of the motor shaft. The central axis of each eccentric portion is an axis that is offset from the axis of the motor shaft by a predetermined eccentric amount.
The reduction-transmission mechanism has a pair of reduction-transmission units provided around the axis of the reduction-transmission mechanism, and a housing that accommodates the reduction-transmission units. The reduction-transmission mechanism is interposed between the electric motor and the differential mechanism, and is coupled to the motor shaft and the differential mechanism (differential case). One of the reduction-transmission units is coupled to the motor shaft, and the other one of the reduction-transmission units is coupled to the differential case.
With the above configuration, the motor shaft of the electric motor is rotated by electric power from the in-vehicle battery, and accordingly the motor torque is transmitted from the electric motor to the differential mechanism via the reduction-transmission mechanism and then distributed to right and left wheels by the differential mechanism.
The reduction-transmission units of the motor torque transmission device of this type have a pair of disc-shaped revolving members, a plurality of outer pins and a plurality of inner pins. The revolving members make revolving motions in accordance with the rotation of the motor shaft of the electric motor. The outer pins apply rotation force to the revolving members. The inner pins are arranged radially inward of the outer pins, and output the rotation force of the revolving members to the differential mechanism as driving force (torque), and the driving force is transmitted to a rotation member at wheel side.
The revolving members each have a center hole and a plurality of pin insertion holes. The revolving members are rotatably supported by the eccentric portions of the motor shaft via bearings (cam-side bearings). The central axis of each center hole coincides with the axis of a corresponding one of the eccentric portions of the motor shaft. The pin insertion holes are arranged at equal intervals around the central axis of each center hole.
The outer pins are arranged at equal intervals around the axis of the motor shaft, and are fitted to the housing of the reduction-transmission mechanism.
The inner pins are passed through the pin insertion holes of the revolving members. The inner pins are arranged at equal intervals on a circle around the axis of the rotation member at wheel side, and are fitted to the differential case. Bearings (pin-side bearings) are fitted to the inner pins. The bearings are used to reduce contact resistance between the inner pins and the inner peripheries which define the pin insertion holes of the revolving members.
However, in the motor torque transmission device described in JP 2007-218407 A, lubricating oil is concentrated on the outer side in the radial direction of the motor shaft by centrifugal force resulting from the revolving motions of the revolving members. This may cause a problem that an amount of lubricating oil near the motor shaft reduces and the lubricating oil is not sufficiently supplied to the bearings located on the inner side in the radial direction of the motor shaft.